Marijuana Doesn’t Make You Stupid

“In particular alcohol use was found to be strongly associated with IQ decline,” the authors write. “No other factors were found to be predictive of IQ change.”

The UK study does find evidence, however, of slightly impaired educational abilities among the very heaviest marijuana users. This group of students scored roughly 3% lower on school exams taken at age 16, even after adjusting for confounding factors.

In a press release accompanying the study, lead author Claire Mokrysz noted that “this is a potentially important public health message- the belief that cannabis is particularly harmful may detract focus from and awareness of other potentially harmful behaviours.” Reviewer Guy Goodwin of Oxford University agreed: “the current focus on the alleged harms of cannabis may be obscuring the fact that its use is often correlated with that of other even more freely available drugs and possibly lifestyle factors. These may be as or more important than cannabis itself.”